Spark Plugs

A typical spark plug consists of a metal shell surrounding a ceramic insulator. A metal electrode extends downward through the center of the insulator and protrudes a small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90and#x00B0; angle so that its tip is even with, and parallel to, the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch) is called the spark plug gap. The spark plug in no way produces a spark but merely provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 40,000 volts or more, which travels to the distributor where it is distributed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and, in so doing, ignites the air/fuel mixture in the combustion chamber.

Fig. Cross-section of a spark plug

Fig. Check the spark plug gap using a filler gauge

spark plug heat range

Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator, the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon, singe it is not hot enough to burn them off. This leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have deposits, but, due to the excessive heat, the electrodes will burn away quickly and in some instances, pre-ignition may result. Pre-ignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the actual spark occurs. This early ignition will usually cause a pinging during low speeds and heavy loads.

The general rule of thumb for choosing the correct heat range when picking a spark plug, is: if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Original equipment plugs are compromise plugs, but most people never have occasion to change their plugs from the factory recommended heat range.

Fig. Spark plug heat range

replacing spark plugs

A set of spark plugs usually requires replacement after 20,000 and 1930,000 miles (32,000 and 1948,0000 km) on cars or trucks with electronic ignition, depending on your style of driving. In normal operation, plug gap increases about 0.001 in. (0.0254mm) for every 1,000 2,500 miles. As the gap increases, the plugs, voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two to three times as much voltage to fire a plug at high speeds than at idle.

When you're removing spark plugs, you should work on one at a time. Don't start by removing the plug wires all at once, because unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape. The best location numbering is near where the wires come out of the cap.

Twist the spark plug boot and remove the boot and wire from the plug. Do not pull on the wire itself, as this will ruin the wire.

Fig. Twist and pull on the rubber boot to remove the spark plug wires; NEVER pull on the wire itself!

If possible, use a brush or rag to clean the area around the spark plug. Make sure that all the dirt is removed so that non will enter the cylinder after the plug is removed.

Remove the spark plug using the proper size socket. Use either a
5
/
8
in. or
13
/
16
in. size socket depending on the engine. Turn the socket counter-clockwise to remove the plug. Be sure to hold the socket straight on the plug to avoid breaking the plug, or rounding off the hex on the plug.

Once the plug is removed, check it's condition for excessive deposits to determine engine condition. This is crucial since plug readings are vital signs of engine condition.

Use a round feeler gauge to check the plug gap. The correct size gauge should pass through the electrode gap with a slight drag. If you're in doubt, try one size smaller and on larger. The smaller gauge should go through easily while the larger one should not go through at all. If the gap is incorrect, use the electrode bending tool on the end of the gauge to adjust the gap. When adjusting the gap, always bend the side electrode. The center electrode is non-adjustable.

Fig. Adjust the electrode gap by carefully bending the side electrode

Squirt a drop of penetrating oil on the threads of the new plug and install it. Don't oil the threads too heavily. turn the plug in clockwise until it is snug.

When the plug is finger-tight, use a torque wrench to tighten it to 15 ft. lbs. (20 Nm), being careful not to overtighten the plug.

Install the plug boot firmly over the plug. Proceed to the next plug.

checking and replacing spark plug cables

Visually inspect the spark plug cables for burns, cuts or breaks in the insulation. Check the spark plug boots and the nipples on the distributor cap and coil. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance.

When installing a new set of spark plug cables, replace the cables one at a time so there will be no mixup. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Insert the nipple firmly into the tower on the distributor cap. Repeat the process for each cable.

Measure the resistance of the spark plug cable with an ohmmeter. Replace the cable is the resistance is not within 9.6 and 1922.5 kilo-ohms.

WARNING

Never kink or pull on the cable. The synthetic conductor may become damage. Never stick is needle or the pointed end of a probe into the cable during a timing check.